Problem: 2 erasers cost $3.38. Which equation would help determine the cost of 10 erasers?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 10 erasers. Since 10 erasers cost $x$ , we have the following proportion: $\dfrac{10}{x}$ We can write the fact that 2 erasers cost $3.38 as a proportion: $\dfrac{2}{\$3.38}$ The cost changes along with the number of erasers purchased, and so the two proportions are equivalent. $\dfrac{10}{x} = \dfrac{2}{\$3.38}$